660 ZOOLOGY sect. 



is afterwards taken by the operculum ; the second develops into 

 the pectines. The four posterior pairs become aborted, though 

 they apparently have some relation to the development of the 

 book-lungs. 



2. Distinctive Characters and Classification. 



The Arachnida are air-breathing Arthropoda in which the body 

 is usually distinguishable into two regions — cephalothorax and 

 abdomen. The cephalothorax bears sessile, usually simple, eyes, 

 two pairs of jointed appendages — the chelicerae and pedipalpi — and 

 four pairs of legs. There are no antennae. The organs of respira- 

 tion, when present, are usually either tracheae or book-lungs, but in 

 the Xiphosura take the form of book -gills. Heart and vascular 

 system are usually present ; the heart is tubular, like that of the 

 Insects. The sexes are nearly always separate, and there is usually 

 no metamorphosis. 



The class is divided into the following orders : — 



Order 1. — Scorpionida. 



Arachnida in which the body consists of a continuous cephalo- 

 thorax and an abdomen, the latter consisting of an anterior 

 broader pre-abdomen of seven segments, and a posterior, narrower 

 post-abdomen of five segments, with a caudal spine in the form 

 of a sting. There are small chelate chelicerse and large chelate 

 pedipalpi. A pair of comb-like pectines occur on the second 

 segment of the pre-abdomen. The organs of respiration are four 

 pairs of book-lungs in the third, fourth, fifth and sixth segments 

 of the pre-abdomen. 



This order includes the Scorpions. 



Order 2. — Pseudoscorpionida. 



Arachnida in which there is a continuous cephalothorax, some- 

 times marked dorsally with two transverse grooves, and a broad 

 abdomen, not divided into pre- and post-abdomen, and not pro- 

 vided with a sting. The chelicerse are very small, the pedipalpi 

 similar to those of the Scorpions. The organs of respiration are 

 a system of tracheae. A pair of spinning-glands are present. 



This order includes the Book-scorpions (Fig. 540). 



Order 3. — Pedipalpida. 



Arachnida in which the body consists of unsegmented cephalo- 

 thorax and flattened abdomen of eleven to twelve segments. 

 The chelicerse are simple, the pedipalpi simple or chelate, and the 



